Atomically thin mica flakes and their application as ultrathin insulating substrates for graphene

We show that it is possible to deposit, by mechanical exfoliation on SiO2/Si wafers, atomically thin mica flakes down to a single monolayer thickness. The optical contrast of these mica flakes on top of a SiO2/Si substrate, which depends on their thickness, the illumination wavelength and the SiO2 substrate thickness, can be quantitatively accounted for by a Fresnel law based model. The preparation of atomically thin insulating crystalline sheets will enable the fabrication of ultrathin defect-free insulating substrates, dielectric barriers or planar electron tunneling junctions. Additionally, we show that few-layer graphene flakes can be deposited on top of a previously transferred mica flake. Our transfer method relies on viscoelastic stamps, as those used for soft lithography. A Raman spectroscopy study shows that such an all-dry deposition technique yields cleaner and higher quality flakes than conventional wet-transfer procedures based on lithographic resists.Comment: 11 pages, 5 figures, 1 graphical abstrac

Electronic spin transport and spin precession in single graphene layers at room temperature

The specific band structure of graphene, with its unique valley structure and Dirac neutrality point separating hole states from electron states has led to the observation of new electronic transport phenomena such as anomalously quantized Hall effects, absence of weak localization and the existence of a minimum conductivity. In addition to dissipative transport also supercurrent transport has already been observed. It has also been suggested that graphene might be a promising material for spintronics and related applications, such as the realization of spin qubits, due to the low intrinsic spin orbit interaction, as well as the low hyperfine interaction of the electron spins with the carbon nuclei. As a first step in the direction of graphene spintronics and spin qubits we report the observation of spin transport, as well as Larmor spin precession over micrometer long distances using single graphene layer based field effect transistors. The non-local spin valve geometry was used, employing four terminal contact geometries with ferromagnetic cobalt electrodes, which make contact to the graphene sheet through a thin oxide layer. We observe clear bipolar (changing from positive to negative sign) spin signals which reflect the magnetization direction of all 4 electrodes, indicating that spin coherence extends underneath all 4 contacts. No significant changes in the spin signals occur between 4.2K, 77K and room temperature. From Hanle type spin precession measurements we extract a spin relaxation length between 1.5 and 2 micron at room temperature, only weakly dependent on charge density, which is varied from n~0 at the Dirac neutrality point to n = 3.6 10^16/m^2. The spin polarization of the ferromagnetic contacts is calculated from the measurements to be around 10%

Charge transport in a single superconducting tin nanowire encapsulated in a multiwalled carbon nanotube

The charge transport properties of single superconducting tin nanowires, encapsulated by multiwalled carbon nanotubes have been investigated by multi-probe measurements. The multiwalled carbon nanotube protects the tin nanowire from oxidation and shape fragmentation and therefore allows us to investigate the electronic properties of stable wires with diameters as small as 25 nm. The transparency of the contact between the Ti/Au electrode and nanowire can be tuned by argonion etching the multiwalled nanotube. Application of a large electrical current results in local heating at the contact which in turn suppresses superconductivity

Measurement of collective dynamical mass of Dirac fermions in graphene

Individual electrons in graphene behave as massless quasiparticles1. Unexpectedly, it is inferred from plasmonic investigations that electrons in graphene must exhibit a non-zero mass when collectively excited. The inertial acceleration of the electron collective mass is essential to explain the behaviour of plasmons in this material, and may be directly measured by accelerating it with a time-varying voltage and quantifying the phase delay of the resulting current. This voltage–current phase relation would manifest as a kinetic inductance, representing the reluctance of the collective mass to accelerate. However, at optical (infrared) frequencies, phase measurements of current are generally difficult, and, at microwave frequencies, the inertial phase delay has been buried under electron scattering. Therefore, to date, the collective mass in graphene has defied unequivocal measurement. Here, we directly and precisely measure the kinetic inductance, and therefore the collective mass, by combining device engineering that reduces electron scattering and sensitive microwave phase measurements. Specifically, the encapsulation of graphene between hexagonal boron nitride layers, one-dimensional edge contacts and a proximate top gate configured as microwave ground together enable the inertial phase delay to be resolved from the electron scattering. Beside its fundamental importance, the kinetic inductance is found to be orders of magnitude larger than the magnetic inductance, which may be utilized to miniaturize radiofrequency integrated circuits. Moreover, its bias dependency heralds a solid-state voltage-controlled inductor to complement the prevalent voltage-controlled capacitor.Physic

Electron spin transport in graphene and carbon nanotubes

Electron spin transport in grafeen en in koolstof nanobuisjes Grafeen, is een kristaal laag van koolstof atomen die slechts één atoomlaag dik is. Een koolstof nanobuisje is te verkrijgen door een grafeen laag op te rollen. In dit proefschrift laten we zien, met behulp van experimenten, dat deze materialen geschikt zijn voor spin transport. Met behulp van electron-beam lithografie hebben we een zogeheten spin valve ('spinklep') gemaakt uit deze materialen. Zo'n spinklep bestaat uit een niet-magnetisch materiaal (grafeen / koolstof nanobuisje) dat ingeklemd zit tussen twee ferromagneten. Vanuit één van de ferromagneten sturen we elektronen met één bepaalde spinrichting het niet-magnetische materiaal in. Ze worden door de ferromagneet aan de andere kant vervolgens weer gedetecteerd. Die tweede ferromagneet laat alleen de elektronen door die ook na beweging door het niet-magnetische materiaal hun oorspronkelijke spinrichting hebben behouden. In die toestand neemt de elektrische weerstand van de spin valve als geheel af. De elektrische weerstand van de spin valve neemt toe op het moment dat we de tweede ferromagneet selectief maken op elektronen met een spin orientatie antiparallel ten opzicht van de geinjecteerde elektronen. Metingen op lage temperaturen (4.2 K) laten zien dat een spinklep uit een grafeen laag, of uit een koolstof nanobuisje, zeer goed werkt en dat de spinrichting behoudend blijft over relatief lange afstanden. Omdat de grafeen-spinklep ook op kamer temperatuur werkt, is grafeen een veelbelovend materiaal voor mogelijke alledaagse toepassingen, zoals in magnetische RAM-geheugens.

Ultrasensitive detection of oligonucleotides: Single-walled carbon nanotube transistor assembled by DNA block copolymer

Biosensors, which harness the unique specific binding properties of biomaterials such as proteis, are increasingly recognized as a powerful tool for chemical sensing. In this context, the detection of nucleic acids DNA and RNA will take on ever more importance for screening as the fields of genomics and diagnostics advance. The same properties that allow molecular recognition-strong, specific non-covalent interactions-also enable bottom-up assembly of sensing architectures. Here, we take advantage of such interactions for the self-assembly of field-effect transistors from semi-conducting single-walled carbon nanotubes selectively dispersed by DNA-block-copolymers and anchored to the electrodes through DNA hybridization. These transistors can sensitively detect the hybridization of complementary target DNA strands through transduction of the chemical recognition event into electrical doping, achieving an analyte sensitivity of 10 fM. Such ultra-sensitive electrical-based detection removes the need for DNA amplification and offers a new route to nucleic acids diagnostics

High Gain Hybrid Graphene-Organic Semiconductor Phototransistors

Hybrid phototransistors of graphene and the organic semiconductor poly(3-hexylthiophene-2,5-diyl) (P3HT) are presented. Two types of phototransistors are demonstrated with a charge carrier transit time that differs by more than 6 orders of magnitude. High transit time devices are fabricated using a photoresist-free recipe to create largearea graphene transistors made out of graphene grown by chemical vapor deposition. Low transit time devices are fabricated out of mechanically exfoliated graphene on top of mechanically exfoliated hexagonal boron nitride using standard e-beam lithography. Responsivities exceeding 10(5) A/W are obtained for the low transit time devices

Quantized conductance of a suspended graphene nanoconstriction

One of the most promising characteristics of graphene(1) is the ability of charge carriers to travel through it ballistically over hundreds of nanometres. Recent developments in the preparation of high mobility graphene(2-4) should make it possible to study the effects of quantum confinement in graphene nanostructures in the ballistic regime. Of particular interest are those effects that arise from edge states, such as spin polarization at zigzag edges(5) of graphene nanoribbons(6,7) and the use of graphene's valley-degeneracy for 'valleytronics'(8). Here we present the observation of quantized conductance(9,10) at integer multiples of 2e(2)/h at zero magnetic field in a high mobility suspended graphene ballistic nanoconstriction. This quantization evolves into the typical quantum Hall effect for graphene at magnetic fields above 60mT. Voltage bias spectroscopy reveals an energy spacing of 8meV between the first two subbands. A pronounced feature at 0.6 x 2e(2)/h present at a magnetic field as low as similar to 0.2 T resembles the '0.7 anomaly' observed in quantum point contacts in a GaAs-AlGaAs two-dimensional electron gas, possibly caused by electron-electron interactions(11)